In-plane switching liquid crystal display device having reflective region and transmissive region

ABSTRACT

A display device includes first and second substrates; a display medium; a pixel electrode; a common electrode; and wherein a voltage is applied to the display medium through the pixel electrode and the common electrode. The common electrode includes at least two branch portions, and a body portion which has substantially the same width as a width of a pixel. The pixel electrode includes at least one branch portion.

This application is a Continuation of Ser. No. 13/370,376, filed Feb.10, 2012, which is a Divisional of Ser. No. 12/373,983, filed Jan. 15,2009 (now U.S. Pat. No. 8,208,102), which is a 371 (national stage) ofPCT/JP2007/056654, filed 28 Mar. 2007, which designates the U.S. andclaims priority to Japanese Application No. 2006-199667, filed 21 Jul.2006, the entire contents of each of which are all hereby incorporatedby reference.

TECHNICAL FIELD

The present invention relates to a display device. More specifically,the present invention relates to a display device preferably used in aliquid crystal display in accordance with In Plane Switching (IPS) modeor Fringe Field Switching (FFS) mode.

BACKGROUND ART

Display devices such as a liquid crystal display device have been widelyused in electronics such as a monitor, a projector, a cellular phone,and a personal digital assistant (PDA) Reflective, transmissive, andtransflective display devices are mentioned as a display type of theliquid crystal display devices. Under relatively dark environments suchas indoor environment, the transmissive liquid crystal display devicewhich provides display using light from a backlight is mainly used.Under relatively bright environments such as outdoor environment, thereflective liquid crystal display device which provides display usingexternal light is mainly used. The transflective liquid crystal displaydevice can provide both of transmissive display and reflective display,and mainly provides transmissive display under indoor environments andprovides reflective display under outdoor environments. Therefore, sucha transflective liquid crystal display device can provide display withhigh qualities under any environments regardless of indoor or outdoorenvironments, and it has been widely equipped with mobile equipment suchas a cellular phone, a PDA, and a digital camera. According to thetransflective liquid crystal display device, for example, a VerticalAlignment (VA) mode is used as display mode. The VA mode is a mode inwhich a liquid crystal molecule is aligned to be vertical to thesubstrate surface when a voltage is not applied and display is carriedout by tilting the liquid crystal molecule by application of a voltage.

However, according to the transflective liquid crystal display device,reflective light passes through the liquid crystal layer twice, buttransmissive light passes through the liquid crystal layer only once.Therefore, if a cell gap is designed to be optimal for reflective light,the transmittance of the transmissive light is about ½ of the optimalvalue. As a solution for this, a method in which the reflective regionand the transmissive region are formed to have different cell gaps toform a multi-gap structure, and the thickness of the liquid crystallayer in the reflective region is decreased is mentioned (for example,refer to Patent Document 1). However, this method needs to formirregularities on the substrate, which complicates the structure on thesubstrate. Further, the production steps of the substrate need to beperformed with high accuracy. Therefore, such a method has room forimprovement. In addition, there is room for improvement also in that theresponse time of the liquid crystal molecule is different between thereflective region and the transmissive region.

IPS mode and FFS mode have been known as display mode of the liquidcrystal display device, in addition to the VA mode. According to the IPSand FFS modes, liquid crystal is operated by a horizontal electric fieldgenerated by a pair of electrodes for driving the liquid crystal, formedon one substrate. According to these systems, the liquid crystalmolecule moves in the horizontal direction (in the direction parallel tothe substrate), which widens the viewing angle. A transflective liquidcrystal display device in IPS mode is disclosed (for example, refer toPatent Document 2). This device in IPS mode also has a multi-gapstructure, and it fails to solve the above-mentioned problems.

Patent Document 1

-   Japanese Kokai Publication No. Hei-11-242226

Patent Document 2

-   Japanese Kokai Publication No. 2005-338264

DISCLOSURE OF INVENTION

The present invention has been made in view of the above-mentioned stateof the art. The present invention has an object to provide a displaydevice which can provide bright display by both of reflective displayand transmissive display without having a multi-gap structure and whichcan reduce a difference in response time between the reflective regionand the transmissive region.

The present inventor made various investigations on a display devicewhich can provide bright display by both of reflective display andtransmissive display without having a multi-gap structure. The inventornoted an arrangement relationship between a pixel electrode and a commonelectrode in the reflective region and the transmissive region. Theinventor found the followings. If, in the horizontal electric field modesuch as IPS mode and FFS mode, each of the pixel electrode and thecommon electrode is provided with a slit; the pixel electrode isprovided with the slit in the reflective region and the transmissiveregion; and the common electrode is provided with the slit substantiallyonly in the reflective region, an intensity of the electric fieldgenerated between the pixel electrode and the common electrode in thereflective region can be made smaller than that in the transmissiveregion without forming the multi-gap structure. As a result, light useefficiency in the reflective display and the transmissive display can beadjusted. As a result, the above-mentioned problems have been admirablysolved, leading to completion of the present invention.

That is, the present invention is a display device including: a pair ofsubstrates; a display medium interposed between the pair of substrates;and a pixel having a reflective region for performing reflective displayand a transmissive region for performing transmissive display, whereinthe display device includes a pixel electrode and a common electrode onone of the pair of substrates, a voltage is applied to the displaymedium through the pixel electrode and the common electrode, each of thepixel electrode and the common electrode is provided with a slit, thepixel electrode is provided with the slit in the reflective region andthe transmissive region, and the common electrode is provided with theslit in the reflective region (hereinafter, also referred to as thefirst display device).

The present invention is mentioned in more detail below.

The first display device of the present invention includes a pair ofsubstrates, a display medium interposed between the pair of substrates,and a pixel having a reflective region for performing reflective displayand a transmissive region for performing transmissive display. In thepresent invention, the kinds of the substrate and the display medium arenot especially limited. For example, according to an active matrixliquid crystal display device, the following embodiment is mentioned.The active matrix liquid crystal display device includes: an activematrix substrate and a color filter substrate as a pair of substrates;and a liquid crystal layer interposed between these substrates as thedisplay medium, wherein on the active matrix substrate, scanning wiringsand signal wirings are arranged to be intersect with each other, and ateach intersection, a TFT that is a switching element is arranged, and onthe color filter substrate, color layers of red (R), green (G), and blue(B) are arranged in each pixel. The liquid crystal display devicegenerally includes a polarizer, a backlight and the like, outside thesubstrates. The reflective display is a mode in which display isperformed by reflecting light outputted from a front light arranged onthe display surface side or external light, inside the display device.The transmissive display is a mode in which display is performed bytransmitting light outputted from the backlight. The size of thereflective region and the transmissive region and the proportion ofthose regions in the pixel are not especially limited. The displaydevice of the present invention is a transflective display devicebecause the reflective region and the transmissive region are arrangedin one pixel.

The display device of the present invention includes a pixel electrodeand a common electrode on one of the substrates, and a voltage isapplied to the display medium through the above-mentioned pixelelectrode and the above-mentioned common electrode. If a voltage isapplied to the pair of electrodes that is the pixel electrode and thecommon electrode, a horizontal electric field parallel to the substrateis generated in the display medium near the pixel electrode and thecommon electrode. This electric field controls the display medium.

In the present invention, each of the pixel electrode and the commonelectrode is provided with a slit; the pixel electrode is provided withthe slit in the reflective region and the transmissive region; and thecommon electrode is provided with the slit in the reflective region.That is, substantially only in the reflective region, the commonelectrode is provided with the slit. In this case, the common electrodemay be provided with the slit also in the transmissive region as long asthe operation and effects of the present invention can be exhibited.Further, an embodiment in which the common electrode is formed over theentire transmissive region is preferable. In the present description,the “common electrode is formed over the entire transmissive region”means that there is no region where the common electrode is not formedon the transmissive region. Thus, according to the display device of thepresent invention, the pixel electrode and the common electrode areconfigured in accordance with IPS mode in the reflective region, and inthe transmissive region, the pixel electrode and the common electrodeare configured in accordance with FFS mode. In the present description,the “IPS mode” means an embodiment in which the slit of the pixelelectrode and the slit of the common electrode are arranged to engagewith each other. Further, the “FFS mode” means an embodiment in whichone of the pixel electrode and the common electrode is provided with theslit and the other is not substantially provided with the slit. If avoltage is applied to such a pair of electrodes that are the pixelelectrode and the common electrode, a horizontal electric field isgenerated between the pixel electrode and the common electrode. Thisintensity of the electric field is decreased as the distance between thepixel electrode and the common electrode is increased. The distancebetween the pixel electrode and the common electrode in the regionhaving an IPS mode configuration can be easily increased than that inthe region having FFS mode configuration because of the configuration ofthe liquid crystal display device. Accordingly, the configuration of thepixel electrode and the common electrode in the reflective region is inaccordance with IPS mode and that in the transmissive region is inaccordance with FFS mode, and thereby the intensity of the electricfield generated between the pixel electrode and the common electrode inthe reflective region can be made smaller than that of the electricfield generated between the pixel electrode and the common electrode inthe transmissive region. The alignment degree of the liquid crystalvaries depending on the electric field intensity, and therefore, byusing this feature, use efficiency of light which passes through theliquid crystal can be adjusted. The shape of the slit of the pixelelectrode and the common electrode is not especially limited as long asa certain width is secured. In addition, the pixel electrode and thecommon electrode are formed in different layers with an insulating filmand the like therebetween because one pixel has both of the IPS modestructure and the FFS mode structure.

As a preferable embodiment of the above-mentioned common electrode, anembodiment in which the common electrode has a comb-tooth shape in thereflective region may be mentioned, for example. If the common electrodehas a comb-tooth shape, a high-density horizontal electric field can begenerated between the pixel electrode and the common electrode, andhence, the display medium can be controlled with high accuracy.

The following embodiments are mentioned as a preferable embodiment of aslit provided for the common electrode. An embodiment in which the slitof the common electrode is entirely surrounded by the common electrode;an embodiment in which the slit of the common electrode has arectangular shape; an embodiment in which the slit of the commonelectrode has a rectangular shape having one bent part; an embodiment inwhich the slit of the common electrode has a zig-zag shape; anembodiment in which the slit of the common electrode has a circular arcshape; and an embodiment in which the slit of the common electrode has ameandering shape. According to such embodiments, a high-densityhorizontal electric field can be generated between the pixel electrodeand the common electrode, and hence, the display medium can becontrolled with high accuracy.

As a preferable embodiment of the above-mentioned pixel electrode, anembodiment in which the pixel electrode has a comb-tooth shape may bementioned, for example. Similarly to the above-mentioned commonelectrode, if the pixel electrode has a comb-tooth shape, a high-densityhorizontal electric field can be generated, and hence, the displaymedium can be controlled with high accuracy.

The following embodiments are mentioned as a preferable embodiment of aslit provided for the pixel electrode. An embodiment in which the slitof the pixel electrode is entirely surrounded by the pixel electrode; anembodiment in which the slit of the pixel electrode has a rectangularshape; an embodiment in which the slit of the pixel electrode has arectangular shape having one bent part; an embodiment in which the slitof the pixel electrode has a zig-zag shape; an embodiment in which theslit of the pixel electrode has a circular arc shape; and an embodimentin which the slit of the pixel electrode has a meandering shape.According to such embodiments, a high-density horizontal electric fieldcan be generated, similarly to the above-mentioned common electrode, andhence, the display medium can be controlled with high accuracy.

As another preferable embodiment of the above-mentioned slit providedfor the pixel electrode, an embodiment in which the slit of the pixelelectrode has substantially the same shape as a shape of the slit of thecommon electrode. According to such an embodiment, the intensity of theelectric field generated at each part where the slit of the commonelectrode is engaged with the slit of the pixel electrode can be madeuniform and as a result, alignment of the liquid crystal can beuniformly controlled. In the present embodiment, the term “the same”means that the same enough to uniform substantially the electric fieldintensity generated at each part (unless display qualities areinfluenced), that is, substantially the same.

As another preferable embodiment of the above-mentioned slit providedfor the pixel electrode, an embodiment in which a width of the slit ofthe pixel electrode in the reflective region is larger than a width ofthe slit of the pixel electrode in the transmissive region is mentioned.The intensity of the electric field generated between the pixelelectrode and the common electrode can be decreased also by increasingthe distance between the pixel electrode and the common electrode. Thus,if this embodiment is adopted in combination with the embodiment of thepresent invention, the intensity of the electric field generated betweenthe pixel electrode and the common electrode in the reflective regioncan be more effectively made smaller than that in the transmissiveregion.

An embodiment in which a shield electrode is arranged between the pixelelectrode and the common electrode in the reflective region is mentionedas a preferable embodiment of the first display device. In the presentdescription, the “shield electrode” means an electrode which ispositioned between the pixel electrode and the common electrode tochange a difference in electric potential between the pixel electrodeand the common electrode. Due to the shield electrode arranged betweenthe pixel electrode and the common electrode, the electric potentialdifference generated between the pixel electrode and the commonelectrode is smaller than that in the case that no shield electrode isarranged. If this embodiment is adopted in combination with theembodiment of the present invention, the intensity of the electric fieldgenerated between the pixel electrode and the common electrode in thereflective region can be more effectively made smaller than that in thetransmissive region. The material for the shield electrode is notespecially limited as long as it has conductivity. A transparentmaterial is particularly preferable. A metal oxide such as indium tinoxide (ITO) is preferably used. The size and shape of the shieldelectrode are not especially limited as long as it can be arrangedbetween the pixel electrode and the common electrode.

It is preferable that the shield electrode is connected to ground. Ifthe shield electrode is connected to ground, a voltage applied to theshield electrode can be maintained at 0V. Further, if the shieldelectrode is connected to ground and thereby an electric potential ofthe shield electrode is 0V, an electric potential difference between thepixel electrode and the common electrode can be effectively decreased.

The present invention is also a display device including: a pair ofsubstrates; a display medium interposed between the pair of substrates;and a pixel having a reflective region for performing reflective displayand a transmissive region for performing transmissive display, whereinthe display device includes a pixel electrode and a common electrode onone of the pair of substrates, a voltage is applied to the displaymedium through the pixel electrode and the common electrode, each of thepixel electrode and the common electrode is provided with a slit, thepixel electrode is provided with the slit in the reflective region, andthe common electrode is provided with the slit in the reflective regionand the transmissive region (hereinafter, also referred to as the seconddisplay device). Thus, the second display device of the presentinvention, in which the common electrode is provided with a slit in bothof the reflective region and the transmissive region and the pixelelectrode is provided with a slit substantially only in the reflectiveregion can exhibit the same effects as in the first display device ofthe present invention, in which the pixel electrode is provided with aslit in both of the reflective region and the transmissive region andthe common electrode is provided with a slit substantially only in thereflective region. Also in the second display device of the presentinvention, it is preferable that the pixel electrode is formed over theentire transmissive region. The shape of the slit of the pixel electrodeand the common electrode is not especially limited as long as a certainwidth is secured. In addition, the pixel electrode and the commonelectrode are formed in different layers with an insulating film and thelike therebetween because one pixel has both of the IPS mode structureand the FFS mode structure.

As a preferable embodiment of the above-mentioned pixel electrode, anembodiment in which the pixel electrode has a comb-tooth shape in thereflective region may be mentioned, for example. Further, the followingembodiments are mentioned as a preferable embodiment of a slit providedfor the pixel electrode. An embodiment in which the slit of the pixelelectrode is entirely surrounded by the pixel electrode; an embodimentin which the slit of the pixel electrode has a rectangular shape; anembodiment in which the slit of the pixel electrode has a rectangularshape having one bent part; an embodiment in which the slit of the pixelelectrode has a zig-zag shape; an embodiment in which the slit of thepixel electrode has a circular arc shape; and an embodiment in which theslit of the pixel electrode has a meandering shape.

Further, as a preferable embodiment of the above-mentioned commonelectrode, an embodiment in which the common electrode has a comb-toothshape may be mentioned, for example. Further, the following embodimentsare mentioned as a preferable embodiment of a slit provided for thecommon electrode. An embodiment in which the slit of the commonelectrode is entirely surrounded by the common electrode; an embodimentin which the slit of the common electrode has a rectangular shape; anembodiment in which the slit of the common electrode has a rectangularshape having one bent part; an embodiment in which the slit of thecommon electrode has a zig-zag shape; an embodiment in which the slit ofthe common electrode has a circular arc shape; and an embodiment inwhich the slit of the common electrode has a meandering shape. Thus, thepreferable embodiments of the second display device of the presentinvention are mentioned. These embodiments are not mentioned in moredetail because, in these embodiments, the common electrode and the pixelelectrode in the preferable embodiments of the first display device ofthe present invention are just replaced with the pixel electrode and thecommon electrode, respectively.

As a preferable embodiment of the above-mentioned slit provided for thecommon electrode, an embodiment in which the slit of the commonelectrode has substantially the same shape as a shape of the slit of thepixel electrode. According to such an embodiment, the intensity of theelectric field generated at each part where the slit of the commonelectrode is engaged with the slit of the pixel electrode can be madeuniform and as a result, alignment of the liquid crystal can beuniformly controlled.

As a preferable embodiment of the above-mentioned slit provided for thecommon electrode, an embodiment in which a width of the slit of thecommon electrode in the reflective region is larger than a width of theslit of the common electrode in the transmissive region is mentioned.According to such an embodiment, the intensity of the electric fieldgenerated between the pixel electrode and the common electrode in thereflective region can be more effectively made smaller than that in thetransmissive region, similarly to the first display device of thepresent invention.

An embodiment in which a shield electrode is arranged between the pixelelectrode and the common electrode in the reflective region is mentionedas a preferable embodiment of the second display device. According tosuch an embodiment, the intensity of the electric field generatedbetween the pixel electrode and the common electrode in the reflectiveregion can be more effectively made smaller than that in thetransmissive region, similarly to the first display device of thepresent invention. Further, it is preferable that the shield electrodeis connected to ground. According to such an embodiment, the voltageadjustment can be more easily performed.

Effect of the Invention

The display device of the present invention can provide bright displayby both of the reflective display and the transmissive display withouthaving a multi-gap structure. In addition, because of the absence of themulti-gap structure, the difference in response time of the liquidcrystal molecule between the reflective region and the transmissiveregion can be decreased.

BEST MODES FOR CARRYING OUT THE INVENTION

The present invention is mentioned in more detail below with referenceto the following Embodiments, but the present invention is not limitedto only these Embodiments.

Embodiment 1

Embodiment 1 shows a liquid crystal display device in accordance withone embodiment of the first display device of the present invention.FIG. 1 is a planar view schematically showing one pixel constituting theliquid crystal display device in Embodiment 1. FIG. 2 is a schematicview showing a cross section taken along dashed line A-B in FIG. 1. Theliquid crystal display device in accordance with Embodiment 1 includesthe first substrate 1, the second substrate 2, and a liquid crystallayer 3 interposed between these substrates, as shown in FIG. 2. Thesecond substrate 2 includes a pixel electrode 4 and a common electrode5, and a voltage is applied to the liquid crystal layer 3 through thepixel electrode 4 and the common electrode 5.

The first substrate 1 includes a color filter layer 6 and the firstalignment film 7 on the liquid crystal layer 3 side in this order. Aglass substrate can be used as the first substrate 1, for example. Thecolor filter layer 6 includes red, green, and blue regions which arearranged in a repeating pattern. The color filter layer 6 may becomposed of regions of four or more colors. Irregularities attributed tothe color filter layer 6 may be flattened by a resin layer forflattening, and the like. The first alignment film 7 determines analignment direction of the liquid crystal layer 3 near the firstalignment film 7.

The second substrate 2 includes a scanning wiring 8, a common wiring(reflector) 9, the first insulating layer 10, a signal wiring 11, a thinfilm transistor 12, the second insulating layer 13, a common electrode5, and the third insulating layer 15 on the liquid crystal layer 3 side,and further includes the pixel electrode 4 and the second alignment film16 on the liquid crystal layer 3 side. A glass substrate can be used asthe second substrate 2, similarly to the first substrate 1. The scanningwiring 8 and the signal wiring 11 are formed in different layers withthe first insulating layer 10 therebetween. Further, the scanning wiring8 and the signal wiring 11 are perpendicular to each other. The thinfilm transistor 12 is positioned near the intersection of the scanningwiring 8 with the signal wiring 11. The thin film transistor 12 has aninverted staggered structure. A gate electrode is connected to thescanning wiring 8; a source electrode is connected to the signal wiring11; and a drain electrode is connected to the pixel electrode 4 throughthe first contact hole 17. A channel part of the thin film transistor 12is formed of an amorphous silicon layer. The common wiring 9 is parallelto the scanning wiring 8 and it is connected to the common electrode 5through the second contact hole 18.

The pixel electrode 4 has a comb-tooth shape in the entire pixel, andthe comb tooth (projection part) is linearly formed. The pixel electrodehas a rectangular slit 19 parallel to the scanning wiring 8. Incontrast, the common electrode 5 has a comb-tooth shape in thereflective region R, but it is formed over the entire transmissiveregion T. Further, the common electrode 5 is positioned in a layer lowerthan the pixel electrode 4 with the third insulating layer 15therebetween. The pixel electrode 4 and the common electrode 5 aretransparent electrodes made of ITO (Indium Tin Oxide). The slit of thecommon electrode 5 has substantially the same shape as a shape of theslit of the pixel electrode 4. According to such a liquid crystaldisplay device in Embodiment 1, a voltage is applied to the pixelelectrode 4 and the common electrode 5, and thereby a horizontalelectric field is generated in the liquid crystal layer 3, which bringsa change in alignment in the liquid crystal layer 3. Thus, light whichpasses through the liquid crystal layer 3 is controlled.

The common wiring 9 protrudes to the display region side and reflectsreflective light 20, as shown in FIG. 2. Transmissive light 21 from abacklight passes through the transmissive region T. According toEmbodiment 1, the wirings such as the common wiring 9 are used as areflector, which provides an effect of decreasing production steps. Thecommon wiring 9 is formed of aluminum with a high reflectance, andthereby brighter reflective display is obtained. Instead of the commonwiring 9, a reflector made of aluminum or a silver alloy, and the like,may be additionally formed. In Embodiment 1, as shown in FIG. 2, thetransmissive region T and the reflective region R are arranged in such away that a boundary between the transmissive region T and the reflectiveregion R is parallel to the short side of the pixel in order to shortenthe boundary.

According to Embodiment 1, the pixel electrode 4 and the commonelectrode 5 are formed of the same material between the transmissiveregion T and the reflective region R. However, the reflective region Rhas an IPS mode structure and the transmissive region T has a FFS modestructure. In such an embodiment, even if the pixel electrode 4 and thecommon electrode 5 are formed of the same material, a voltage which isapplied to the liquid crystal layer 3 is different in intensity betweenthe transmissive region T and the reflective region R. Accordingly, bothof the reflective display and the transmissive display can be performedwithout providing the liquid crystal layer 3 with a multi-gap structureby additionally forming a step-forming layer in the reflective region R.The second alignment film 16 is further provided on the liquid crystallayer 3 side of the pixel electrode 4. The second alignment film 16determines the alignment direction of the liquid crystal layer 3 nearthe second alignment film 16.

FIGS. 3, 4, and 5 each show an arrangement relationship among thepolarizer, the retarder, and the liquid crystal molecule. FIG. 3 showsan arrangement relationship among the polarizer, the retarder, and theliquid crystal molecule under no voltage application. FIG. 4 shows anarrangement relationship among the polarizer, the retarder, and theliquid crystal molecule in the reflective region under voltageapplication. FIG. 5 shows an arrangement relationship among thepolarizer, the retarder, and the liquid crystal molecule in thetransmissive region under voltage application.

According to Embodiment 1, as shown in FIG. 2, the first polarizer 22 isarranged on the side opposite to the liquid crystal layer 3 of the firstsubstrate 1, and the second polarizer 23 is arranged on the sideopposite to the liquid crystal layer 3 of the second substrate 2. Thefirst and second polarizers 22 and 23 are arranged in such a way that atransmission axis 26 of the first polarizer 22 is perpendicular to atransmission axis 27 of the second polarizer 23. The first retarder 24is arranged between the first substrate 1 and the first polarizer 22.The second retarder 25 is arranged between the second substrate 2 andthe second polarizer 23.

As shown in FIG. 3, the first retarder 24 has a retardation of ¼wavelength, and the phase delay axis 28 is set to make an angle of 45°with the alignment direction of the liquid crystal molecule 30 in theclockwise direction. The transmission axis 26 of the first polarizer 22is set to be parallel to the alignment direction of the liquid crystalmolecule 30. The second retarder 25 has a retardation of ¼ wavelength.The second retarder 25 is arranged in such a way that the phase delayaxis 29 is perpendicular to the phase delay axis 28 of the firstretarder 24.

In the reflective region R, if a voltage of less than a threshold isapplied to the pixel electrode 4 and the common electrode 5, a stackedbody composed of the liquid crystal layer 3, the first polarizer 22 andthe first retarder 24 functions as a circular polarizer. A linearpolarized light which has passed through the first polarizer 22 isconverted into a circularly-polarized light after passing through thefirst retarder 24. Then, the circularly-polarized light is reflected bythe reflector and converted into a counter-rotating circularly-polarizedlight. When entering the first polarizer 22 again, the light isconverted into a linear polarized light whose oscillation direction isvertical to the transmission axis 26 of the first polarizer 22.Therefore, the linear polarized light is absorbed by the first polarizer22 and dark display is obtained. If a voltage more than a threshold isapplied to the pixel electrode 4 and the common electrode 5, as shown inFIG. 4, the alignment of the liquid crystal molecule 30 is changed by aspecific angle θ in the clockwise direction. As a result, incident lightis reflected by the reflector 14, and then when entering the firstpolarizer 22 again, the light is converted into linear polarized lightwhose oscillation direction is parallel to the transmission axis of thefirst polarizer 22. Therefore, the light is not absorbed by the firstpolarizer 22 and bright display is obtained.

In the transmissive region T, the first retarder 24 is perpendicular tothe second retarder 25. Therefore, a retardation in the normal directionof the first substrate 1 is zero, and the display in this direction isnot influenced. If a voltage of less than a threshold is applied to thepixel electrode 4 and the common electrode 5, the long axis of theliquid crystal molecule 30 is perpendicular to the transmission axis 27of the second polarizer 23. Therefore, the linear polarized light whichhas passed through the second polarizer 23 is a linear polarized lightvertical to the transmission axis 26 of the first polarizer 22.Therefore, the light is absorbed by the first polarizer 22, and darkdisplay is obtained. If a voltage of more than a threshold is applied tothe pixel electrode 4 and the common electrode 5, as shown in FIG. 5,the alignment of the liquid crystal molecule 30 is changed by a specificangle 2θ in the clockwise direction. When entering the first polarizer22, the light is converted into a linear polarized light whoseoscillation direction is parallel to the transmission axis 26 of thefirst polarizer 22. Therefore, the light is not absorbed by the firstpolarizer 22, and bright display is obtained.

The first and second retarders 24 and 25 are made of a material whichhardly shows wavelength dispersion of refractive index, for example, anorbornene material (product of JSR Corp., trade name: ARTON). In such acase, darker display which is less colored can be obtained.

Thus-prepared transflective liquid crystal display panel is connected toa driving device, and a backlight is arranged on the back face of thepanel, for example. Thus, a transflective liquid crystal display deviceis completed.

A modified embodiment of Embodiment 1 is mentioned below.

According to the present embodiment, the comb tooth (projection part) ofthe pixel electrode 4 and the common electrode 5 may not have a linearshape shown in FIG. 1. For example, it may have a shape shown in FIGS. 6to 11. According to the comb-tooth-shaped electrode 31 (either or bothof the pixel electrode 4 and the common electrode 5) shown in FIG. 6,the comb tooth has a V shape which has one bent part at the middle ofthe comb tooth, like a broken line. The slit of the comb-tooth-shapedelectrode 31 has a rectangular shape having one bent part. The combtooth of the comb-tooth-shaped electrode 32 shown in FIG. 7 has two bentparts, like a broken line. The entire of the comb tooth has asubstantially V shape. The slit of the comb-tooth-shaped electrode 32has a rectangular shape having two bent parts. With regard to thecomb-tooth-shaped electrode 33 shown in FIG. 8, the comb tooth has threebent parts, like a broken line. The entire of the comb tooth has a shapecomposed of two substantially V shapes. The slit of thecomb-tooth-shaped electrode 33 has a rectangular shape having three bentparts, that is, a zig-zag shape. With regard to the comb-tooth-shapedelectrode 34 shown in FIG. 9, the comb tooth has a circular-arc shapewhich has a curved part at the middle of the comb tooth. The slit of thecomb-tooth-shaped electrode 34 also has a circular-arc shape. Withregard to the comb-tooth-shaped electrode 35 shown in FIG. 10, the combtooth has a circular-arc shape which has three curved parts. The entireof the comb tooth has a shape composed of two substantially V shapes.The slit of the comb-tooth-shaped electrode 35 has a meandering shape.The pixel electrode 4 may not have a comb-tooth shape and it may have ashape shown in FIG. 11, in which a rectangular slit 36 is entirelysurrounded by the pixel electrode.

FIG. 12 is a schematic view showing the pixel electrode 4 and the commonelectrode 5 in Embodiment 1. FIG. 12( a) is a planar view schematicallyshowing the pixel electrode 4 and the common electrode 5. FIGS. 12( b)and 12(c) are schematic views each showing a cross section taken alongdashed line C-D in FIG. 12( a). As shown in FIG. 12( a), the pixelelectrode 4 is formed to have a comb-tooth shape in both of thetransmissive region T and the reflective region R. Further, the commonelectrode is formed to have a comb-tooth shape in the reflective regionR, but it is formed over the entire transmissive region T. Thearrangement relationship of the cross section between the pixelelectrode 4 and the common electrode 5 is not especially limited to theembodiment in which the pixel electrode 4 is formed in a layer closer tothe liquid crystal layer 3 than the common electrode 5, as shown in FIG.12( b). As shown in FIG. 12( c) the common electrode 5 may be formed ina layer closer to the liquid crystal layer 3 than the common electrode4.

Embodiment 2

Embodiment 2 shows a liquid crystal display device in accordance withone embodiment of the first display device of the present invention.FIG. 13 is a planar view schematically showing the pixel electrode 4 andthe common electrode 5 constituting the liquid crystal display device inaccordance with Embodiment 2. The liquid crystal display device inEmbodiment 2 is the same as that in Embodiment 1, except that as shownin FIG. 13, the width of the slit of the pixel electrode 4 in thereflective region R is larger than that of the slit of the pixelelectrode 4 in the transmissive region T. The intensity of the electricfield generated between the pixel electrode 4 and the common electrode 5can be decreased also by increasing a distance between the pixelelectrode 4 and the common electrode 5. Therefore, if this embodiment isadopted in combination with the embodiment in which one pixel has bothof the FFS mode structure and the IPS mode structure, the intensity ofthe electric field generated between the pixel electrode 4 and thecommon electrode 5 can be more effectively decreased in the reflectiveregion R than that in the transmissive region T.

Embodiment 3

Embodiment 3 shows a liquid crystal display device in accordance withone embodiment of the first display device of the present invention.FIG. 14 is a schematic view showing the pixel electrode 4 and the commonelectrode 5 constituting the liquid crystal display device in accordancewith Embodiment 3. FIG. 14( a) is a planar view schematically showingthe pixel electrode 4 and the common electrode 5. FIG. 14( b) is aschematic view showing a cross section taken along dashed line E-F inFIG. 14( a). The liquid crystal display device in Embodiment 3 is thesame as that in Embodiment 1, except that as shown in FIG. 14, theshield electrode 50 is arranged between the comb tooth of the pixelelectrode 4 and the comb tooth of the common electrode 5 in thereflective region R. The shield electrode 50 is arranged in thereflective region R as shown in FIG. 14( a), and further arrangedbetween the layer where the pixel electrode 4 is arranged and the layerwhere the common electrode 5 is arranged as shown in FIG. 14( b). Thus,the intensity of the electric field generated between the pixelelectrode 4 and the common electrode 5 can be decreased also byarranging the shield electrode 50 between the pixel electrode 4 and thecommon electrode 5. Therefore, if this embodiment is adopted incombination with the embodiment in which one pixel has both of the FFSmode structure and the IPS mode structure, the intensity of the electricfield generated between the pixel electrode 4 and the common electrode 5can be more effectively decreased in the reflective region R than thatin the transmissive region T. In addition, it is preferable that theshield electrode 50 is connected to ground.

Embodiment 4

Embodiment 4 shows a liquid crystal display device in accordance withone embodiment of the second display device of the present invention.FIG. 15 is a planar view schematically showing the pixel electrode 4 andthe common electrode 5 constituting the liquid crystal display device inaccordance with Embodiment 4. According to Embodiment 4, as shown inFIG. 15, the common electrode 5 is formed to have a comb-tooth shape inthe entire pixel. Further, the pixel electrode 4 is formed to have acomb-tooth shape in the region R, but it is formed over the entiretransmissive region T. That is, according to Embodiment 4, the structureof the pixel electrode 4 and the structure of the common electrode 5 maybe counterchanged in the above-mentioned embodiments. Even in such anembodiment, the effects of the present invention can be exhibitedbecause the FFS mode structure and the IPS mode structure are formed inone pixel.

The present application claims priority under the Paris Convention andthe domestic law in the country to be entered into national phase onPatent Application No. 2006-199667 filed in Japan on Jul. 21, 2006, theentire contents of which are hereby incorporated by reference.

The term “or more” in the present description means that the valuedescribed (boundary value) is included.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a planar view schematically showing one pixel constituting theliquid crystal display device in accordance with Embodiment 1.

FIG. 2 is a schematic view showing a cross section taken along dashedline A-B shown in FIG. 1.

FIG. 3 is a schematic view showing arrangement relationship among thepolarizer, the retarder, and the liquid crystal molecule during novoltage application in accordance with Embodiment 1.

FIG. 4 is a schematic view showing arrangement relationship among thepolarizer, the retarder, and the liquid crystal molecule in thereflective region during voltage application in accordance withEmbodiment 1.

FIG. 5 is a schematic view showing arrangement relationship among thepolarizer, the retarder, and the liquid crystal molecule in thetransmissive region during voltage application in accordance withEmbodiment 1.

FIG. 6 is a planar view schematically showing electrodes (the pixelelectrode and the common electrode) in accordance with a modifiedembodiment (the rectangular slit has one bent part) of Embodiment 1.

FIG. 7 is a planar view schematically showing electrodes (the pixelelectrode and the common electrode) in accordance with a modifiedembodiment (the rectangular slit has two bent parts) of Embodiment 1.

FIG. 8 is a planar view schematically showing electrodes (the pixelelectrode and the common electrode) in accordance with a modifiedembodiment (the rectangular slit has three bent parts) of Embodiment 1.

FIG. 9 is a planar view schematically showing electrodes (the pixelelectrode and the common electrode) in accordance with a modifiedembodiment (the slit has a circular arc shape) of Embodiment 1.

FIG. 10 is a planar view schematically showing electrodes (the pixelelectrode and the common electrode) in accordance with a modifiedembodiment (the slit has a meandering shape) of Embodiment 1.

FIG. 11 is a planar view schematically showing electrodes (the pixelelectrode and the common electrode) in accordance with a modifiedembodiment (the slit is entirely surrounded by the electrodes) ofEmbodiment 1.

FIG. 12 is a schematic view showing the pixel electrode and the commonelectrode (the slit has a rectangular shape) in accordance withEmbodiment 1. FIG. 12( a) shows a schematic planar view. FIGS. 12( b)and 12(c) are schematic views each showing a cross section taken alongdashed line C-D in FIG. 12( a).

FIG. 13 is a planar view schematically showing the pixel electrode andthe common electrode constituting the liquid crystal display device inaccordance with Embodiment 2.

FIG. 14 is a schematic view showing the pixel electrode and the commonelectrode constituting the liquid crystal display device in accordancewith Embodiment 3. FIG. 14( a) is a schematic planar view. FIG. 14( b)is a schematic view showing a cross section taken along dashed line E-Fin FIG. 14( a).

FIG. 15 is a planar view schematically showing the pixel electrode andthe common electrode constituting the liquid crystal display device inaccordance with Embodiment 4.

EXPLANATION OF NUMERALS AND SYMBOLS

-   1: The first substrate-   2: The second substrate-   3: Liquid crystal layer-   4: Pixel electrode-   5: Common electrode-   6: Color filter layer-   7: The first alignment film-   8: Scanning wiring-   9: Common wiring (reflector)-   10: The first insulating layer-   11: Signal wiring-   12: Thin film transistor-   13: The second insulating layer-   15: The third insulating layer-   16: The second alignment film-   17: The first contact hole-   18: The second contact hole-   19: Slit-   20: Reflective light-   21: Transmissive light-   22: The first polarizer-   23: The second polarizer-   24: The first retarder-   25: The second retarder-   26: Transmission axis of the first polarizer-   27: Transmission axis of the second polarizer-   28: Phase delay axis of the first retarder-   29: Phase delay axis of the second retarder-   30: Liquid crystal molecule-   31: Comb-tooth-shaped electrode (the rectangular slit has one bent    part)-   32: Comb-tooth-shaped electrode (the rectangular slit has two bent    parts)-   33: Comb-tooth-shaped electrode (the rectangular slit has three bent    parts)-   34: Comb-tooth-shaped electrode (the slit has a circular arc shape)-   35: Comb-tooth-shaped electrode (the slit has a meandering shape)-   36: Electrode (the slit is entirely surrounded by the electrode)-   50: Shield electrode-   T: Transmissive region-   R: Reflective region

The invention claimed is:
 1. A display device comprising: first andsecond substrates; a display medium interposed between the first andsecond substrates; a pixel electrode, and a common electrode configuredto apply a voltage to the display medium in cooperation with the pixelelectrode, wherein the pixel electrode includes: a first portionextending along a first direction and having a first width with respectto a second direction perpendicular to the first direction, and aplurality of second portions of the pixel electrode being electricallyconnected to the first portion, each of the plurality of second portionsextends along the first direction and has a second width, which issmaller than the first width, with respect to the second direction;wherein a third portion of the common electrode is provided betweenadjacent two of the plurality of second portions of the pixel electrodewhen viewed in plane; wherein the first portion of the pixel electrodeoverlaps with the common electrode when viewed in plane, and theplurality of second portions of the pixel electrode do not overlap withthe common electrode when viewed in plane.
 2. The device of claim 1,wherein the plurality of the second portions of the pixel electrode andthe third portion of the common electrode are located in a reflectiveregion for performing reflective display.
 3. A display devicecomprising: first and second substrates; a display medium interposedbetween the first and second substrates; a pixel electrode, and a commonelectrode configured to apply a voltage to the display medium incooperation with the pixel electrode, wherein the pixel electrodeincludes: a first portion extending along a first direction and having afirst width with respect to a second direction perpendicular to thefirst direction, and a second portion of the pixel electrode beingelectrically connected to the first portion, the second portion extendsalong the first direction and has a second width, which is smaller thanthe first width, with respect to the second direction; the commonelectrode includes a plurality of third portions, each of the pluralityof third portions extends along the first direction; the second portionof the pixel electrode being provided between adjacent two of theplurality of third portions of the common electrode when viewed inplane; wherein the first portion of the pixel electrode overlaps withthe common electrode when viewed in plane, and the second portion of thepixel electrode does not overlap with the common electrode when viewedin plane.
 4. The device of claim 3, wherein the second portion of thepixel electrode and the plurality of the third portions of the commonelectrode are located in a reflective region for performing reflectivedisplay.